In the task manager it will not show avast directly, there are different components, for example the update component that you can find in the task manager it called aswUpdSv.exe, with that said I would terminate all procceses starting with ash in the taskmanager. The only component of avast! that I noticed that does not start with ash is the update component which is aswUpd.exe. After that restart your system, if that does not fix your problem do a reinstall/repair of avast! antivirus.